Primary Lymphoid Tissue
Bone Marrow
- Bone marrow is found on the inside of long bones
- The shaft of long bones has three layers – periosteum, compact bone and then the inner medullary cavity lined by endosteum and containing bone marrow.
- The bone marrow is the origin of all immune cells.
- Most cells (including B lymphocytes) develop here.
Thymus
- The thymus is an organ with two lobes located behind the sternum within the mediastinum.
- The thymus gland is where lymphocytes develop into T cells.
- The outer area is called the cortex, which is where T cells proliferate and start develop into specialised cells
- The inner area is called the medulla, which is where the mature T cells are found before they enter the blood, and it also contains the waste products of defective T cells
Lymphatic System
- This is a network of lymphatic vessels, similar to blood vessels
- There is no built in pump (like the heart)
- Circulation relies on muscles surrounding the vessels to pump
- Contain valves to direct the flow of lymph
- Eventually drains into the circulation via the
- Thoracic duct (entering between the left subclavian and internal jugular veins) – this is the larger of the two ducts
- Right lymphatic duct (entering at right subclavian or internal jugular veins)
- The lymphatic system contains a fluid called lymph
- This is interstitial fluid that have drained from the intersitial space
- High concentration of lymphocytes and dendritic cells
- Low protein content
- No red blood cells or other leukocytes
Lymph Nodes
- Structures that occur intermittently along the course of lymphatic vessels
- Afferent vessels drain in to lymph nodes
- Efferent vessel drains away from lymph nodes
- Their role is to:
- Hold lots of cells of the immune system, particularly lymphocytes.
- Act as filters for pathogens and abnormal cells
- Shaped a bit like a kidney
- Surrounded by a capsule
- Have their own blood supply
- Have three areas
- Cortex – contains B cells and follicles
- Paracortex – contains T cells and dendritic cells
- Medulla – contains plasma cells, T and B cells and macrophages
- Hilum – leading to the efferent vessel and where the blood vessels enter
- Follicles are areas where groups of B cells clump together and wait to be activated
- Primary follicles contain unstimulated B cells and do not have germinal centres
- Secondary follicles have germinal centres (that stain a lighter colour) and contain activated B cell that are generating plasma cells and memory B cells
Spleen
- The spleen is an organ made up of
- Red pulp
- White pulp
- Located in the left upper quadrant of the abdomen
- Surrounded by a capsule
- Red pulp is where old red blood cells are removed (filter through a web of reticular fibres that destroy old RBCs but young ones can squeeze through without getting destroyed).
- White pulp contains the immune system
- Immune cells wrap themselves around the arterioles in the spleen forming the white pulp.
- Immediately around the arteriole is the “peri-arteriolar lymphoid sheath” (PALS)
- This contains T cells and dendritic cells
- The marginal zone surrounds the PALS
- Contains B cells and macrophages
- At intervals along the arteriole there are follicles
- Primary follicles contain unstimulated B cells
- Germinal centres are areas within the primary follicles where B cells have been stimulated and are differentiating into plasma cells and secreting antibodies.
Mucosa-Associated Lymphoid Tissue
- This is lymphoid tissue associated with the mucosa of the GI, respiratory and urinary tracts.
- It contains primary follicles and germinal centres similar to the spleen and lymph nodes
- Much of it is unstructured and appears randomly along the tract
- Examples of structured MALT tissue is:
- Tonsils
- Adenoids
- Appendix
- Peyer’s patches (in the ileum of the small intestine)